Bing:
The FAA has never had a refractive error limit on any of its medical certificates. Ever.
Back in the 1920's, the FAA's predecessor agency had set a distant visual acuity limit of 20/50 correctable to 20/20 for First- and Second-Class medicals. The Third-Class originally had a rather bizarre standard: the applicant could have 20/50 vision without glasses — and not necessarily even correctible to anything better! — and not be required to use corrective lenses, but if his DVA were worse than 20/50, then it had to be correctable to 20/30! The US standards imposed for 1st-/2d-Class certificates in those days actually represented a far stricter set of requirements than ICAO, since individuals with –3.00D refractive errors generally [though not always] have less than 20/50 uncorrected.
When the ICAO standards were imposed in 1946, the US Civil Aviation Agency simply ignored them and continued with its own stricter requirements. When the FAA came along to supercede the CAA in 1958, the same standards were kept; the only change that was made at that time was to retitle "CAR Part 3" as "FAR Part 67". The uncorrected DVA limit was lowered to 20/100 during the late 1960's. Waivers were routinely granted for those with DVA's of 20/200 or better, and it was not unheard of for applicants with 20/400 vision — or worse — to be approved. The uncorrected DVA limit was wisely dropped altogether in 1996.
WRT the reasoning behind the refractive error limit, this was borrowed directly from the military, who originally imposed limits at/before the turn of the 20th Century, not for medical reasons, but for logistical reasons! The idea was that if an army had to issue glasses to its troops, then a requirement that recruits not have spherical/cylindrical errors exceeding an arbitrarily imposed limit would reduce the myriad number of prescription lens blanks to a [using a term dear to the hated beancounters now!] "more manageable level". I.e.: since standard lenses are manufactured in graduations of 0.25 dioptres, a ±3.00 limit means less difficulty keeping all of the possible prescriptions in stock than, for example, ±5.00. As a matter of trivia, current US Army refraction limits for initial entry (all recruits, not just aviation personnel) are ±8.00.
So there you have it.... Don't blame the medical profession for starting this nonsense. It's really the beancounters' fault! <G>